Disc brakes for vehicles are known to comprise, essentially, a disc integral with a wheel, to which disc two gaskets or "pads" of friction material, such as lining, for example, are associated for position on opposite sides of the disc and able to be activated by means of corresponding coaxial hydraulic pistons, the latter being suitably associated to a hydraulic operating circuit so as to cause, under braking condition, the clamping of the disc, operated by the pads simultaneously pushed by the respective pistons, and the dissipation of kinetic energy by virtue of the friction exerted by the same pads on the disc of each wheel.
Also known are conventional disc brakes in which the pads are in constant contact with the disc surface, that is, even when the pistons are deactivated. This implies a constant and unwanted dissipation of kinetic energy during the running of the vehicle, which causes and reduction of mechanical efficiency of the vehicle and results exceedingly detrimental in case of bicycles, especially of race model, inasmusch as the cyclist must exert an additional effort to win the thus provided resistance to the rotation of the wheel.